Gas turbine engines are commonly used for numerous different purposes and applications. Unless stated otherwise, the term “gas turbine engine” is used herein and in the claims to refer generally to the well known type of internal combustion rotary engine which comprises: an air intake; an air intake compressor; a combustor; a power turbine; and an exhaust. All of these components are typically contained within a single engine housing. The engine compressor typically employs a series of rotating and stationary compressor blades to compact and pressurize the intake air. In the combustor, fuel is added to the pressurized air and ignited whereby the hot combustion gas formed in the combustor then expands through stationary nozzles and moves at high velocity into the turbine section. In the turbine section, the energy from the high velocity combustion gas is converted into useful rotational power through the expansion of the heated compressed gas over a series of turbine rotor blades. This rotational power is then, for example, delivered to driven equipment through a rotating power output shaft, typically via a speed reduction gearbox. The hot engine exhaust can be discharged to the atmosphere or used for other purposes such as, e.g., heat recovery for various purposes, cogeneration systems utilizing waste heat steam generation to power a conventional steam turbine in a combined cycle configuration with the gas turbine, etc.
In addition to being used as propulsion engines for aircraft, watercraft, and land vehicles, gas turbine engines are also used as industrial engines for numerous purposes including, but not limited to: electrical power generation; oil and gas production, processing, transporting, and pipeline transmission; and as direct mechanical power drive systems in many other applications.
Industrial gas turbine engine systems are employed for electrical power generation in distributed industrial and central utility station power plants, industrial processing facilities, and in all manner of public and private buildings and institutions. Said systems can be used as base load systems, peak shaving systems, combined heat and power (e.g., cogeneration) systems, stand-by or emergency systems, distributive power systems, etc.
The fuel combusted in gas turbine engines provides the chemical energy that is converted into shaft power, which powers the engine compressor, drives the load and is exhausted. Typically, gas turbine engines use a single fuel. The fuel will often be, for example, the fluid that is being compressed or pumped in a pipeline by a compressor or pump driven by the engine. Many times alternative sources of energy could be used to provide the needed shaft power, with lower overall cost, but there is currently no method of incorporating these sources into gas turbine based power systems.
An example of a prior art gas turbine engine system 2 for electrical power generation is illustrated in FIG. 1. The prior art system 2 is a combined heat and power system comprising: a gas turbine engine 4 having an engine housing 5; an air inlet filter 6 through which air flows to the air intake 7 of the engine housing 5; an electrical power generator 8 which is driven by the engine 4; a supplemental burner 10 wherein all uncombusted or partially combusted materials remaining in the engine exhaust are burned; a heat recovery steam generator 12 or other heat recovery system which recovers heat energy from the hot exhaust stream; and an exhaust bypass 14 and silencer 16 which can optionally be used to deliver the engine exhaust directly to the atmosphere rather than through the heat recovery system 12.
In the oil and gas industry, common uses of industrial gas turbine engine systems include, but are not limited to: (a) driving centrifugal gas compressors or reciprocating compressors for gas pipeline transmission, natural gas gathering, gas storage and withdrawal, gas lifting, and export sales gas; (b) as power for pump drive systems for transporting crude oil, transporting other liquids, water flood operations, etc.; (c) as compressor drive systems for other operations such as air compression, refrigeration, etc.; and (d) electrical power generation for offshore platforms, FPS power generation, gas production and processing facilities, etc.
An example of a prior art gas turbine powered system 20 for compressing and/or transmitting natural gas is illustrated in FIG. 2. The prior art gas compression system 20 comprises: a gas turbine engine 4 of the same type used in the system of FIG. 1; a turbine air inlet duct 24 provided with an air intake filter (not shown); a turbine exhaust duct 26; and a centrifugal gas compressor 28 which is driven by the turbine power shaft 30.
Gas turbine engines are commonly in use in gas transmission pipeline stations throughout the United States and the rest of the world. The U.S. interstate natural gas pipeline network relies on more than 1,200 natural gas compressor stations to maintain the continuous flow of natural gas from supply areas to consumers. The purpose of each compressor station is to boost the pressure in the natural gas pipeline and move the gas further downstream. Such stations are typically situated between 50 and 100 miles apart along the length of a gas pipeline system and are designed for continuous pipeline operation. The average station is capable of moving about 700 million cubic feet of natural gas per day, while the largest can move as much as 4.6 billion cubic feet per day.
Although mainline natural gas compressor stations vary widely in size and layout, the basic components of such stations include compressor units, scrubber/filters, cooling facilities, emergency shutdown systems, and computerized flow control and dispatch systems that maintain the operational integrity of the stations. Similar compressor stations are also used at underground natural gas storage sites for injection and withdrawal operations and in production areas where operational wellhead pressures are not always enough to move the flow into the high-pressure mainline or gathering header systems.
Most natural gas compressor stations are unmanned and monitored by an off-site supervisory control and data acquisition system that manages and coordinates the operations of several compressor stations linked together in a natural gas pipeline system. Almost all mainline compressor stations have multiple compressor units. The onsite computerized flow control system will typically manage these units so that only the appropriate number of units required to meet current flow requirements are operating at any given time, thus attempting to optimize operational efficiency and extend the life of the equipment.
An example of a prior art gas pipeline compressor station 40 is illustrated in FIG. 3. The prior art compressor station 40 comprises: a pipeline gas intake flow line 42 from the gas mainline; scrubbers and filters 44 for the gas stream; a plurality of turbine engine driven gas compressor systems 46, with turbine engines 4 driving compressors 28 of the same type shown in the system of FIG. 2, which can be operated one at a time and/or in various parallel combinations; an alternative or additional series arrangement 48 comprising two turbine driven compressors 28 operating in series with an inter-stage cooler 54 for cooling the partially compressed natural gas between the two compressors 28; after cooler systems 56 for cooling the compressed natural gas before returning the repressurized gas to the main line 60; and an offsite supervised control and data acquisition system 62 that manages and coordinates the operations of the several compressor systems 46 and/or 48.
As will be understood by those in the art, pumping station systems similar to the compressor station system shown in FIG. 3 are used for pumping crude oil, refined products and other fluids through liquid pipelines. The primary differences being that, in the liquid pumping station, the turbine engines will be used to drive pumps and no after cooling will be required.
When used in the oil and gas, power generation, and other types of applications discussed above, gas turbine engines are relied upon to provide a steady power output, oftentimes when other power producing equipment is operating at maximum capability, such as during periods of hot weather. Also, in many oil and gas applications, power generation applications, etc., gas turbine engines must be installed and operated at significant elevations above sea level. Unfortunately, however, at higher ambient air temperatures and/or higher elevations, the power output of a gas turbine engine will be significantly less than the rated power output of the engine at ISO conditions (i.e., operation at sea level with an ambient air temperature of 15° C. (59° F.)).
Because, during operation, the internal air compressor of a gas turbine engine runs within a specific speed range, or at a constant speed, the volume of intake air flowing into a gas turbine engine will be essentially constant, regardless of ambient conditions and air density. As a result, the mass of the air received by the engine will vary with density and have a significant impact upon the power output provided.
As mentioned above, the rated power output for a gas turbine engine is typically based upon operation at sea level with an ambient air temperature of 15° C. (59° F.) and a relative humidity of 60% (i.e., ISO conditions). However, when operating, for example, at an altitude of 3,000 feet above sea level and an ambient air temperature of 100° F., the significantly lower air density encountered under these conditions will typically reduce the resulting power output of the engine to as little as 35% of its rated power output or less. (On the other hand, it is important to note for purposes of the present invention that the power curve for a gas turbine engine is such that the engine's power output can also significantly exceed the rated power output when the engine is operated at air density conditions (e.g., sea level operation at cold winter temperatures) wherein the air is more dense than at ISO conditions).
Consequently, when significantly lower air density conditions are encountered due to higher ambient temperature and/or higher elevations, it has been necessary heretofore to add additional fuel to the engine in order to compensate for the lower air mass flow through the engine and thereby increase the power output. Unfortunately, in addition to other disadvantages and harmful effects, the increased engine fuel-to-air ratio increases the engine combustion temperature and thus operates to: (a) increase the levels of thermally generated nitrous oxide (NOx) and other greenhouse gas emissions discharged in the engine exhaust, (b) test the metallurgical temperature limits of the engine components, (c) accelerate and increase the need for engine maintenance and part replacement, and (d) reduce the life of the engine.
Moreover, because of the significant variability in available power output which can occur on a seasonal, daily, or even hourly basis due to significant changes in ambient air conditions, it has been necessary heretofore to compensate for such periods of low available power output by adding and operating additional engines. However, this approach is very costly in additional fuel and maintenance expense, and results in higher total emissions, and largely does not address directly the poor efficiency, high combustion temperature, and emissions problems experienced when operating gas turbine engines at air densities significantly lower than at the original ISO design conditions.
Also, in addition to the need for an effective and efficient solution to these problems, a need exists for systems, methods, and improvements which will allow existing turbine engines already in the field to meet increasingly stringent governmental emissions standards and requirements. Industrial gas turbine engines of the type discussed above are typically constructed to provide decades of reliable service if properly maintained. In fact, in gas pipeline transmission operations, the majority of mainline compressor stations currently in operation are at least 30 years old. However, although these existing systems still have many remaining years of useful life, the ability to continue to operate these engines, without extensive internal combustion system modifications and having to pay significant emissions related penalties, is very much in question. Current trends include replacing gas turbine powered compressors with electrical motor driven compressors, to escape emissions penalties.
Unfortunately, the solutions offered heretofore by turbine engine manufacturers for improving emissions have focused on replacing existing turbine engines with new low NOx version turbine engine systems. However, as will be apparent, the capital cost of replacing all existing industrial turbine engine systems in order to comply with current emissions requirements would be excessive.
As for prior efforts to address swings in power availability due to varying ambient temperatures, these have involved various forms of inlet air cooling, aimed at increasing density, which, unfortunately, have burdened the engine with higher parasitic operating loads and costs.
Moreover, to compensate for reduced gas turbine power at high altitude stations, the practice in the industry has simply been to use larger gas turbines with the associated higher capital and operating costs. Similarly, the only way to incorporate alternative sources of power into gas turbine based power systems has been to add additional driven loads within the station.